The environment in the core of a nuclear BWR is demanding for the components positioned therein. The environment is for example highly oxidative. One of the consequences of this demanding environment inside the core of a nuclear BWR is that the channel box of the fuel assemblies may be distorted. The channel box may for example bulge or bow. Channel box bow is due to elongation of one channel box side relative the opposite channel box side. Channel box bow is known to arise for different reasons, e.g. initial manufacturing, residual stress relaxation under irradiation, differential irradiation growth and shadow corrosion.
The problem of shadow corrosion on components comprising zirconium based alloys in the core of a nuclear BWR has been known for a long time. Shadow corrosion is a local corrosion enhancement and can appear on a zirconium based alloy component when the component is in close contact with another metal. Referring to the above, shadow corrosion on the outer side of a channel box can occur when a control rod blade is inserted next to the channel box, i.e. when the channel box consisting of a zirconium based alloy is in close contact with a control rod blade usually having an outer surface of stainless steel.
Shadow corrosion early in the life of a fuel assembly, i.e. shadow corrosion on the fuel assembly due to an inserted control rod next to the fuel assembly during the first several months of operation, is generally believed to drive the problem of enhanced channel bow of the channel boxes in a nuclear BWR. The shadow corrosion can result in increased absorbed hydrogen-induced growth of the outer side of the channel box being closest to the control rod. The increased absorbed hydrogen-induced growth can lead to bowing of the channel box towards the control rod late in the life of the fuel assembly. The bow of the channel box towards the control rod may lead to channel box-control rod interference, which may for example cause the fuel assemblies to lift due to friction when the control rods are inserted into the core.
Studies have shown that shadow corrosion strongly depends on the distance between the zirconium based alloy component and the component comprising another metal. The occurrence of shadow corrosion is therefore most significant in the case of a large control rod blade and a small distance between the control rod blade and the channel box.
JP 05-323069 discloses a channel box for a nuclear BWR, wherein the channel box has axially projecting pads on the outer sides of the channel box. The projecting pads are provided on the two outer sides of the channel box that faces a control rod when the control rod is inserted into the core of the reactor. The object of the projecting pads is to ensure a gap between the fuel assemblies, where the control rod is to be inserted, even if the channel box is deformed by channel box bowing against the control rod. The bowing of the channel box is described to be caused by elongation of the channel box due to exposure to neutrons during operation. Accordingly, even if the channel box would bow towards the control rod, the projecting pads on the outer sides of the channel box will ensure that it is possible to insert a control rod between the fuel assemblies.